Network Working Group P. Eronen
Internet-Draft Nokia
Intended status: Standards Track H. Tschofenig
Expires: December 27, 2010 Nokia Siemens Networks
Y. Sheffer
Independent
June 25, 2010
An Extension for EAP-Only Authentication in IKEv2draft-ietf-ipsecme-eap-mutual-05.txt
Abstract
IKEv2 specifies that EAP authentication must be used together with
public key signature based responder authentication. This is
necessary with old EAP methods that provide only unilateral
authentication using, e.g., one-time passwords or token cards.
This document specifies how EAP methods that provide mutual
authentication and key agreement can be used to provide extensible
responder authentication for IKEv2 based on methods other than public
key signatures.
Note to RFC Editor: this document updates
draft-ietf-ipsecme-ikev2bis, and therefore depends on that document.
Please add "Updates: RFCxxxx" to the title page, where "xxxx" is the
RFC number assigned to IKEv2-bis.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 27, 2010.
Copyright Notice
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in [RFC4072],
is an authentication framework which supports multiple authentication
mechanisms. Today, EAP has been implemented at end hosts and routers
that connect via switched circuits or dial-up lines using PPP
[RFC1661], IEEE 802 wired switches [IEEE8021X], and IEEE 802.11
wireless access points [IEEE80211i].
One of the advantages of the EAP architecture is its flexibility.
EAP is used to select a specific authentication mechanism, typically
after the authenticator requests more information in order to
determine the specific authentication method to be used. Rather than
requiring the authenticator (e.g., wireless LAN access point) to be
updated to support each new authentication method, EAP permits the
use of a backend authentication server which may implement some or
all authentication methods.
IKEv2 ([RFC4306] and [I-D.ietf-ipsecme-ikev2bis]) is a component of
IPsec used for performing mutual authentication and establishing and
maintaining security associations for IPsec ESP and AH. In addition
to supporting authentication using public key signatures and shared
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secrets, IKEv2 also supports EAP authentication.
IKEv2 provides EAP authentication since it was recognized that public
key signatures and shared secrets are not flexible enough to meet the
requirements of many deployment scenarios. By using EAP, IKEv2 can
leverage existing authentication infrastructure and credential
databases, since EAP allows users to choose a method suitable for
existing credentials, and also makes separation of the IKEv2
responder (VPN gateway) from the EAP authentication endpoint (backend
AAA server) easier.
Some older EAP methods are designed for unilateral authentication
only (that is, EAP peer to EAP server). These methods are used in
conjunction with IKEv2 public key based authentication of the
responder to the initiator. It is expected that this approach is
especially useful for "road warrior" VPN gateways that use, for
instance, one-time passwords or token cards to authenticate the
clients.
However, most newer EAP methods, such as those typically used with
IEEE 802.11i wireless LANs, provide mutual authentication and key
agreement. Currently, IKEv2 specifies that these EAP methods must
also be used together with public key signature based responder
authentication.
In order for the public key signature authentication of the gateway
to be effective, a deployment of PKI is required, which has to
include management of trust anchors on all supplicants. In many
environments, this is not realistic, and the security of the gateway
public key is the same as the security of a self-signed certificate.
Mutually authenticating EAP methods alone can provide a sufficient
level of security in many circumstances, and in fact in some
deployments, IEEE 802.11i uses EAP without any PKI for authenticating
the WLAN access points.
This document specifies how EAP methods that offer mutual
authentication and key agreement can be used to provide responder
authentication in IKEv2 completely based on EAP.
1.1. Terminology
All notation in this protocol extension is taken from [RFC4306].
Numbered messages refer to the IKEv2 message sequence when using EAP.
Thus:
o Message 1 is the request message of IKE_SA_INIT.
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o Message 2 is the response message of IKE_SA_INIT.
o Message 3 is the first request of IKE_AUTH.
o Message 4 is the first response of IKE_AUTH.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Scenarios
In this section we describe two scenarios for extensible
authentication within IKEv2. These scenarios are intended to be
illustrative examples rather than specifying how things should be
done.
Figure 1 shows a configuration where the EAP and the IKEv2 endpoints
are co-located. Authenticating the IKEv2 responder using both EAP
and public key signatures is redundant. Offering EAP based
authentication has the advantage that multiple different
authentication and key exchange protocols are available with EAP with
different security properties (such as strong password based
protocols, protocols offering user identity confidentiality and many
more).
+------+-----+ +------------+
O | IKEv2 | | IKEv2 |
/|\ | Initiator |<---////////////////////--->| Responder |
/ \ +------------+ IKEv2 +------------+
User | EAP Peer | Exchange | EAP Server |
+------------+ +------------+
Figure 1: EAP and IKEv2 endpoints are co-located
Figure 2 shows a typical corporate network access scenario. The
initiator (client) interacts with the responder (VPN gateway) in the
corporate network. The EAP exchange within IKE runs between the
client and the home AAA server. As a result of a successful EAP
authentication protocol run, session keys are established and sent
from the AAA server to the VPN gateway, and then used to authenticate
the IKEv2 SA with AUTH payloads.
The protocol used between the VPN gateway and AAA server could be,
for instance, Diameter [RFC4072] or RADIUS [RFC3579]. See Section 6
for related security considerations.
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+-------------------------------+
| Corporate network |
| |
+-----------+ +--------+ |
| IKEv2 | AAA | Home | |
IKEv2 +////----->+ Responder +<---------->+ AAA | |
Exchange / | (VPN GW) | (RADIUS/ | Server | |
/ +-----------+ Diameter) +--------+ |
/ | carrying EAP |
| | |
| +-------------------------------+
v
+------+-----+
o | IKEv2 |
/|\ | Initiator |
/ \ | VPN client |
User +------------+
Figure 2: Corporate Network Access
3. Solution
IKEv2 specifies that when the EAP method establishes a shared secret
key, that key is used by both the initiator and responder to generate
an AUTH payload (thus authenticating the IKEv2 SA set up by messages
1 and 2).
When used together with public key responder authentication, the
responder is in effect authenticated using two different methods: the
public key signature AUTH payload in message 4, and the EAP-based
AUTH payload later.
If the initiator does not wish to use public key based responder
authentication, it includes an EAP_ONLY_AUTHENTICATION notification
payload (type TBD-BY-IANA) in message 3. The Protocol ID and SPI
size fields are set to zero, and there is no additional data
associated with this notification.
If the responder supports this notification and chooses to use it, it
omits the public key based AUTH payload and CERT payloads from
message 4.
If the responder does not support the EAP_ONLY_AUTHENTICATION
notification or does not wish to use it, it ignores the notification
payload, and includes the AUTH payload in message 4. In this case
the initiator MUST verify that payload and any associated
certificates, as per [RFC4306].
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protected identity information may be sent between the EAP endpoints.
This third, optional property of the method provides protection
against certain types of attacks (see Section 6.2 for an
explanation), and therefore in some scenarios, methods that allow for
channel binding are to be preferred. It is noted that at the time of
writing, even when such capabilities are provided, they are not fully
specified in an interoperable manner. In particular, no RFC
specifies what identities should be sent under the protection of the
channel binding mechanism, or what policy is to be used to correlate
identities at the different layers.
5. IANA considerations
This document defines a new IKEv2 Notification Payload type,
EAP_ONLY_AUTHENTICATION, described in Section 3. This payload must
be assigned a new type number from the "status types" range.
6. Security Considerations
Security considerations applicable to all EAP methods are discussed
in [RFC3748]. The EAP Key Management Framework [RFC5247] deals with
issues that arise when EAP is used as a part of a larger system.
6.1. Authentication of IKEv2 SA
It is important to note that the IKEv2 SA is not authenticated by
just running an EAP conversation: the crucial step is the AUTH
payload based on the EAP-generated key. Thus, EAP methods that do
not provide mutual authentication or establish a shared secret key
MUST NOT be used with the modifications presented in this document.
6.2. Authentication with separated IKEv2 responder/EAP server
As described in Section 2, the EAP conversation can terminate either
at the IKEv2 responder or at a backend AAA server.
If the EAP method is terminated at the IKEv2 responder then no key
transport via the AAA infrastructure is required. Pre-shared secret
and public key based authentication offered by IKEv2 is then replaced
by a wider range of authentication and key exchange methods.
However, typically EAP will be used with a backend AAA server. See
[RFC5247] for a more complete discussion of the related security
issues; here we provide only a short summary.
When a backend server is used, there are actually two authentication
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exchanges: the EAP method between the client and the AAA server, and
another authentication between the AAA server and IKEv2 gateway. The
AAA server authenticates the client using the selected EAP method,
and they establish a session key. The AAA server then sends this key
to the IKEv2 gateway over a connection authenticated using, e.g.,
IPsec or TLS.
Some EAP methods do not have any concept of pass-through
authenticator (e.g., NAS or IKEv2 gateway) identity, and these two
authentications remain quite independent of each other. That is,
after the client has verified the AUTH payload sent by the IKEv2
gateway, it knows that it is talking to SOME gateway trusted by the
home AAA server, but not which one. The situation is somewhat
similar if a single cryptographic hardware accelerator, containing a
single private key, would be shared between multiple IKEv2 gateways
(perhaps in some kind of cluster configuration). In particular, if
one of the gateways is compromised, it can impersonate any of the
other gateways towards the user (until the compromise is discovered
and access rights revoked).
In some environments it is not desirable to trust the IKEv2 gateways
this much (also known as the "Lying NAS Problem"). EAP methods that
provide what is called "connection binding" or "channel binding"
transport some identity or identities of the gateway (or WLAN access
point/NAS) inside the EAP method. Then the AAA server can check that
it is indeed sending the key to the gateway expected by the client.
A potential solution is described in
[I-D.arkko-eap-service-identity-auth], and see also
[I-D.clancy-emu-aaapay].
In some deployment configurations, AAA proxies may be present between
the IKEv2 gateway and the backend AAA server. These AAA proxies MUST
be trusted for secure operation, and therefore SHOULD be avoided when
possible; see Sec. 2.3.4 of [RFC4072] Sec. 4.3.7 of [RFC3579] for
more discussion.
6.3. Protection of EAP payloads
Although the EAP payloads are encrypted and integrity protected with
SK_e/SK_a, this does not provide any protection against active
attackers. Until the AUTH payload has been received and verified, a
man-in-the-middle can change the KEi/KEr payloads and eavesdrop or
modify the EAP payloads.
In IEEE 802.11i wireless LANs, the EAP payloads are neither encrypted
nor integrity protected (by the link layer), so EAP methods are
typically designed to take that into account.
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In particular, EAP methods that are vulnerable to dictionary attacks
when used in WLANs are still vulnerable (to active attackers) when
run inside IKEv2.
The rules in Section 4 are designed to avoid this potential
vulnerability.
6.4. Identities and Authenticated Identities
When using this protocol, each of the peers sends two identity
values:
1. An identity contained in the IKE ID payload.
2. An identity transferred within the specific EAP method's
messages.
(IKEv2 omits the EAP Identity request/response pair, see Sec. 3.16 of
[I-D.ietf-ipsecme-ikev2bis].) The first identity value can be used
by the recipient to route AAA messages and/or to select
authentication and EAP types. But it is only the second identity
that is directly authenticated by the EAP method. The reader is
referred to Sec. 2.16 of [I-D.ietf-ipsecme-ikev2bis] regarding the
need to base IPsec policy decisions on the authenticated identity.
In the context of the extension described here, this guidance on
IPsec policy applies both to the authentication of the client by the
gateway and vice versa.
6.5. User identity confidentiality
IKEv2 provides confidentiality for the initiator identity against
passive eavesdroppers, but not against active attackers. The
initiator announces its identity first (in message 3), before the
responder has been authenticated. The usage of EAP in IKEv2 does not
change this situation, since the ID payload in message 3 is used
instead of the EAP Identity Request/Response exchange. This is
somewhat unfortunate since when EAP is used with public key
authentication of the responder, it would be possible to provide
active user identity confidentiality for the initiator.
IKEv2 protects the responder's identity even against active attacks.
This property cannot be provided when using EAP. If public key
responder authentication is used in addition to EAP, the responder
reveals its identity before authenticating the initiator. If only
EAP is used (as proposed in this document), the situation depends on
the EAP method used (in some EAP methods, the server reveals its
identity first).
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Implemented IESG review comments from David Harrington and Adrian
Farrel. In particular, this document updates
[I-D.ietf-ipsecme-ikev2bis]. Added a paragraph on interaction with
IKE session resumption.
A.2. -04
Anti-nit.
A.3. -03
Implemented IETF LC comments from Dan Harkins and Tero Kivinen.
A.4. -02
Implemented several WGLC comments. EAP methods are required to be
resistant to dictionary attacks to be used here.
A.5. -01
List of proposed EAP methods is now informative, not normative.
A.6. draft-ietf-ipsecme-mutual-auth-00
Initial WG draft, based on draft-eronen-ipsec-ikev2-eap-auth-07, with
the following changes: if the responder does not support this
mechanism, the initiator reverts to normal RFC 4306 behavior; the
initiator must abort immediately if it doesn't like the proposed EAP
method; allowed EAP methods are explicitly listed.
Appendix B. Alternative Approaches
In this section we list alternatives which have been considered
during the work on this document. We concluded that the solution
presented in Section 3 seems to fit better into IKEv2.
B.1. Ignore AUTH payload at the initiator
With this approach, the initiator simply ignores the AUTH payload in
message 4 (but obviously must check the second AUTH payload later!).
The main advantage of this approach is that no protocol modifications
are required and no signature verification is required. A
significant disadvantage is that the EAP method to be used cannot be
selected to take this behavior into account.
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The initiator could signal to the responder (using a notification
payload) that it did not verify the first AUTH payload.
B.2. Unauthenticated public keys in AUTH payload (message 4)
Another solution approach suggests the use of unauthenticated public
keys in the public key signature AUTH payload (for message 4).
That is, the initiator verifies the signature in the AUTH payload,
but does not verify that the public key indeed belongs to the
intended party (using certificates)--since it doesn't have a PKI that
would allow this. This could be used with X.509 certificates (the
initiator ignores all other fields of the certificate except the
public key), or "Raw RSA Key" CERT payloads.
This approach has the advantage that initiators that wish to perform
certificate-based responder authentication (in addition to EAP) may
do so, without requiring the responder to handle these cases
separately. A disadvantage here, again, is that the EAP method
selection cannot take into account the incomplete validation of the
responder's certificate.
If using RSA, the overhead of signature verification is quite small,
compared to the g^xy calculation required by the Diffie-Hellman
exchange.
B.3. Using EAP derived session keys for IKEv2
It has been proposed that when using an EAP method that provides
mutual authentication and key agreement, the IKEv2 Diffie-Hellman
exchange could also be omitted. This would mean that the session
keys for IPsec SAs established later would rely only on EAP-provided
keys.
It seems the only benefit of this approach is saving some computation
time (g^xy calculation). This approach requires designing a
completely new protocol (which would not resemble IKEv2 anymore) we
do not believe that it should be considered. Nevertheless, we
include it for completeness.
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